4-Alkoxy cyclohexane-1-amino-carboxylic acid esters and method for the production thereof

ABSTRACT

The invention relates to novel 4-alkoxy-cyclohexane-1-amino-carboxylic esters of the formula (IV) 
                         
in which R 1  represents OR 3 , R 2  represents alkyl, and R 3  represents alkyl, to intermediates and processes for their preparation, and to their use as intermediates in the synthesis of insecticidal, acaricidal, and herbicidal compounds or pharmaceutically active compounds.

The invention relates to novel 4-alkoxy-cyclohexane-1-amino-carboxylicesters, to intermediates and processes for their preparation and totheir use as intermediates in the synthesis of insecticidal, acaricidaland herbicidal compounds or pharmaceutically active compounds.

Substituted cyclic aminocarboxylic acids can generally be obtained bythe Bucherer-Bergs synthesis or by the Strecker synthesis, resulting ineach case in different isomeric forms. Thus, using the conditions of theBucherer-Bergs synthesis in the preparation of the substituted cyclicaminocarboxylic acids of the general formula (I)

give predominantly the isomer (I-a),

in which the radical R¹ and the amino group are arranged cis to oneanother, whereas the conditions of the Strecker synthesis givepredominantly the trans isomer (I-b)

(J. Chem. Soc. 1961, 4372–4379; Chem. Pharm. Bull. 21 (1973) 685–691;Chem. Pharm. Bull. 21 (1973) 2460–2465; Can. J. Chem. 53 (1975)3339–3350).

The Bucherer-Bergs reaction is generally carried out by reacting asubstituted cyclic ketone of the general formula (II)

in a solvent or solvent mixture with ammonium carbonate and an alkalimetal cyanide, generally sodium cyanide or potassium cyanide, followedby isolation of the resulting hydantoin of the general formula (III)

Here, the hydantoins of the general formula (III) are usually obtainedas mixtures of the cis isomers (III-a)

and trans isomers (III-b)

The hydantoins of the general formula (III) are subsequently hydrolysedby known methods, under acidic or alkaline conditions, to give thesubstituted cyclic aminocarboxylic acids of the general formula (I).

The substituted cyclic aminocarboxylic acids of the general formula (I)can then be esterified by known methods of organic chemistry to give thesubstituted cyclic aminocarboxylic esters of the general formula (IV)

We have found novel compounds of the formulae (IV-a) and (IV-b)

in which

-   -   R¹ represents OR³,    -   R² represents alkyl and    -   R³ represents alkyl.

Preference is given to compounds of the formulae (IV-a) and (IV-b), inwhich

-   -   R¹ represents OR³,    -   R² represents C₁–C₆-alkyl and    -   R³ represents C₁–C₄-alkyl.

Particular preference is given to compounds of the formulae (IV-a) and(IV-b) in which

-   -   R¹ represents OR³,    -   R² represents methyl, ethyl, n-propyl or n-butyl and    -   R³ represents methyl, ethyl, n-propyl, n-butyl or i-butyl.

Some compounds (for example from EP-A-596298; WO 95/20572, EP-A-668267;WO 95/26954; WO 96/25395; WO 96/35664; WO 97/02243; WO 97/01535; WO97/36868; WO 98/05638) require substituted cyclic aminocarboxylic estersof the general formula (IV) as precursors.

For certain of these compounds disclosed, for example, in EP-A-596298;WO 95/20572; EP-A-668267; WO 95/26954; WO 96/25395; WO 96/35664; WO97/02243; WO 97/01535; WO 97/36868; WO 98/05638, a preparation with theuse of substituted cyclic aminocarboxylic esters of the general formula(IV) in which the cis isomer (IV-a) is the only or at least thepredominant isomer may be advantageous.

Solvents used for the Bucherer-Bergs reaction are, in general,approximately 50% strength aqueous methanol (J. Org. Chem. 53 (1988)4069–4074) or approximately 50% strength aqueous ethanol (J. Chem. Soc.1961, 4372–4379; Chem. Pharm. Bull. 21 (1973) 685–691; Chem. Pharm.Bull. 21 (1973) 2460–2465; Can. J. Chem. 53 (1975) 3339–3350; Can. J.Chem. 57 (1979) 1456–1461). In optimized Bucherer-Bergs reactions, too,the solvent used was aqueous ethanol (J. Heterocycl. Chem. 21 (1984)1527–1531). A further solvent known for the Bucherer-Bergs reaction isN,N-dimethylformamide (Helv. Chim. Acta 67 (1984) 1291–1297). However,if these solvents are used for preparing the hydantoins of the generalformula (III), unsatisfactory yields are obtained. Moreover, theisolated products are contaminated considerably by inorganic fractions.Additional purification operations result in products havingcompositions which vary considerably with respect to cis and transisomers, so that a constant product quality cannot be ensured.

It has been found that compounds of the formula (III)

in which

-   -   R¹ is as defined above,    -   are obtained by reacting compounds of the formula (II)

in which

-   -   R¹ is as defined above        with ammonium carbonate and alkali metal cyanides or        trimethylsilyl cyanide (TMSCN) in the solvent water.

Surprisingly, by the process according to the invention, the compoundsof the formula (III) can be obtained in high yield and purity and with ahigh and reproducible proportion of the cis isomer (III-a)

in which

-   -   R¹ represents OR³,        where    -   R³ represents alkyl.

In the general formulae (II), (III) and (III-a), the radical

-   -   R¹ represents OR³,        where    -   R³ preferably represents C₁–C₄-alkyl.

Particularly preferably, R³ represents methyl, ethyl, n-propyl, n-butylor i-butyl.

Very particularly preferably, R³ represents methyl.

Emphasis is given to the compound of the formula (III-a), in which R³represents methyl.

The compounds of the formula (III) and the isomers of the formulae(III-a) and (III-b) are novel and form part of the subject-matter ofthis invention.

In the general formula (II-b), the variable R¹ is as defined above.

Compounds of the formula (III) can be hydrolysed by known methods togive the compounds of the formula (I)

in which

-   -   R¹ is as defined above        and then esterified by known methods to compounds of the formula        (IV).

Preferred alkali metal cyanides which can be used for preparing thecompounds of the formula (III) are lithium cyanide, sodium cyanide andpotassium cyanide; particular preference is given to sodium cyanide andpotassium cyanide.

Based on the ketone, the amount of alkali metal cyanide or TMSCN is from0.9 to 3 mol per mole of ketone. Preference is given to using amountsfrom 1 to 2.5 mol per mole of ketone; particular preference is given toamounts from 1.1 to 2 mol of alkali metal cyanide per mole of ketone.

The amount of ammonium carbonate is from 0.5 to 7 mol of ammoniumcarbonate per mole of ketone. Preference is given to using amounts from0.8 to 5 mol per mole of ketone; particular preference is given toamounts from 1 to 5 mol of ammonium carbonate per mole of ketone.

The reaction temperature for the process according to the invention isfrom 20 to 100° C.; preference is given to a temperature range from 30to 70° C.

It is also possible to carry out the reaction under elevated or reducedpressure.

The reaction product is isolated in a simple manner by filtering thereaction mixture and drying the filter residue. The filtration iscarried out at a temperature of from 0 to 40° C., preferably at atemperature of from 15 to 30° C.

In this manner, the desired hydantoins of the formula (III) are obtainedin high yield and purity, with a reproducible isomer ratio.

The process according to the invention can be illustrated, for example,by the scheme below:

This invention also provides a process for preparing the compounds ofthe formula (III-a)

in which

-   -   R¹ is as defined above,        characterized in that compounds of the formula (II),

in which

-   -   R¹ is as defined above    -   are reacted with an alkali metal cyanide and ammonium carbonate        in water.

Particular preference is given to a process for preparing the compoundof the formula (III-a), in which

-   -   R¹ represents OR³,        where    -   R³ represents methyl,        characterized in that 4-methoxycyclohexanone is reacted with an        alkali metal cyanide and ammonium carbonate in water.

Suitable for use as alkali metal cyanides are lithium cyanide, sodiumcyanide or potassium cyanide; preference is given to sodium cyanide andpotassium cyanide. Particular preference is given to sodium cyanide.

Based on the compound of the formula (II), the amount of alkali metalcyanide is from 0.9 to 3 mol per mole of the compound of the formula(II). Preference is given to amounts of from 0.9 to 2.5 mol per mole ofthe compound of the formula (II); particular preference is given toamounts of from 1 to 2 mol of alkali metal cyanide per mole of thecompound of the formula (II).

At the same time, the amount of ammonium carbonate is from 0.8 to 2 molof ammonium carbonate per mole of the compound of the formula (II).Preference is given to using amounts of from 1 to 1.8 mol per mole ofthe compound of the formula (II).

The amount of the solvent water is from 500 to 3000 ml of water per moleof the compound of the formula (II); preference is given to an amount ofwater of from 1000 to 2500 ml per mole of the compound of the formula(II).

The reaction temperature for the process according to the invention isfrom 20 to 100° C.; preference is given to a temperature range of from30 to 70° C.

The reaction product is isolated in a simple manner by filtering thereaction mixture and drying the filter residue. The filtration iscarried out at a temperature of from 0 to 40° C., preferably at atemperature of from 0 to 20° C.

This invention also provides a process for isolating the compound of theformula (III-a),

in which

-   -   R¹ is as defined above,        characterized in that compounds of the formula (III) (cis/trans        mixtures (III-a)/(III-b)) are treated with aqueous ammonia, and        the solid which remains undissolved is isolated in a known        manner.

Based on the trans isomer of the formula (III-b) present in the mixture,the amount of ammonia is from 1 to 30 mol per mole of the trans isomerof the formula (III-b). Preference is given to amounts of from 4 to 20mol per mole of the trans isomer of the formula (III-b); particularpreference is given to amounts of from 6 to 15 mol of ammonia per moleof the trans isomer of the formula (III-b).

The amount of the solvent water is from 500 to 3000 ml of water per moleof the compound of the formula (III); preference is given to an amountof water of from 1000 to 2500 ml per mole of the compound of the formula(III).

The temperature for the process according to the invention is from 0 to100° C.; preference is given to a temperature range of from 10 to 60° C.

The hydantoins of the general formula (III) can be hydrolysed by knownmethods to the amino acids of the general formula (I), which can then beesterified by known methods to give compounds of the formula (IV).

The present invention also provides substituted cyclic aminocarboxylicacids of the general formula (I)

in which

-   -   R¹ represents OR³,        where    -   R³ represents alkyl, preferably C₁–C₄-alkyl.

The substituted cyclic aminocarboxylic acids of the general formula (I)can be present either as mixtures of the cis isomers (I-a) and transisomers (I-b), or as pure isomers.

The compounds of the formula (I) are novel and form part of thesubject-matter of this invention.

Particular preference is given to compounds of the general formula (I)in which

-   -   R¹ represents OR³,        where    -   R³ represents methyl or ethyl.

Very particular preference is given to compounds of the general formula(I-a), in which

-   -   R¹ represents OR³,        where    -   R³ represents methyl or ethyl.

Substituted cyclic aminocarboxylic acids of the formula (I) oraminocarboxylic esters of the formula (IV) are intermediates in thepreparation of other compounds which are used, for example, as activecompounds in plant protection or as pharmaceutically active compounds.

Thus, for example, EP-A-596 298, WO 95/20572, EP-A-668 267, WO 95/26954,WO 96/25395, WO 96/35664, WO 97/02243, WO 97/01535, WO 97/36868, WO98/05638 disclose that substituted cyclic aminocarboxylic acids arerequired for preparing substituted phenylketoenols which can be used aspesticides and herbicides.

The subject-matter of the invention is illustrated by the examplesbelow, without limiting it in any way.

PREPARATION EXAMPLES Comparative Example 1

26.9 g [280 mmol] of ammonium carbonate and 5.88 g [120 mmol] of sodiumcyanide are initially charged in 110 ml of water. Starting at roomtemperature, a solution of 7.7 g [60 mmol] of 4-methoxy-cyclohexanone in110 ml of ethanol is added dropwise. The reaction mixture is stirred at55–60° C. for 16 hours and then concentrated completely (according toHPLC, the cis/trans ratio is 66:34). The crude product is stirred with100 ml of 50% strength aqueous ethanol for 1 hour, cooled to 0–5° C.,stirred at 0–5° C. for 1 hour and filtered. The filter residue is dried,giving 12.07 g of a solid having a product content of 57.8% (HPLC,compared to standard), resulting in a yield of 58.7% of theory; thecis/trans ratio is 91:9. Elemental analysis shows a sodium content of16%.

Comparative Example 2

The procedure of Comparative Example 1 was repeated. Following work-up,a product having a cis/trans ratio of 80:20 was obtained.

Example 1

134.6 g [1.4 mol] of ammonium carbonate and 29.4 g [0.6 mol] of sodiumcyanide are initially charged in 560 ml of water. Starting at roomtemperature, 38.5 g [0.3 mol] of 4-methoxy-cyclohexanone are addeddropwise. The reaction mixture is stirred at 55–60° C. for 16 hours,cooled to 0–5° C. and stirred at this temperature for 2 hours. The solidis filtered off with suction and dried. This gives 57.88 g of a solidhaving a product content of 93.4% (HPLC, compared to standard),resulting in a yield of 90.9% of theory; the cis/trans ratio is 71:29.Elemental analysis shows a sodium content of 1.2%.

Example 2

134.6 g [1.4 mol] of ammonium carbonate and 22.05 g [0.45 mol] of sodiumcyanide are initially charged in 560 ml of water. Starting at roomtemperature, 38.5 g [0.3 mol] of 4-methoxy-cyclohexanone are addeddropwise. The reaction mixture is stirred at 55–60° C. for 4 hours,cooled to 0–5° C. and stirred at this temperature for 2 hours. The solidis filtered off with suction and dried. This gives 57.64 g of a solidhaving a product content of 93.7% (HPLC, compared to standard),resulting in a yield of 90.8% of theory; the cis/trans ratio is 72:28.Elemental analysis shows a sodium content of 1.3%.

Example 3

134.6 g [1.4 mol] of ammonium carbonate and 16.17 g [0.33 mol] of sodiumcyanide are initially charged in 560 ml of water. Starting at roomtemperature, 38.5 g [0.3 mol] of 4-methoxy-cyclohexanone are addeddropwise. The reaction mixture is stirred at 55–60° C. for 4 hours,cooled to 0–5° C. and stirred at this temperature for 2 hours. The solidis filtered off with suction and dried. This gives 61.02 g of a solidhaving a product content of 94.1% (HPLC, compared to standard),resulting in a yield of 96.5% of theory; the cis/trans ratio is 71:29.

Example 4

The procedure of Example 3 is repeated. This gives 59.54 g of a solidhaving a product content of 93.6% (HPLC, compared to standard),resulting in a yield of 93.7% of theory; the cis/trans ratio is 71:29.

Example 5

134.6 g [1.4 mol] of ammonium carbonate and 16.17 g [0.33 mol] of sodiumcyanide are initially charged in 560 ml of water. Starting at roomtemperature, 38.5 g [0.3 mol] of 4-methoxy-cyclohexanone are addeddropwise. The reaction mixture is stirred at 55–60° C. for 4 hours andthen stirred at room temperature overnight. At room temperature thesolid is filtered off with suction and dried. This gives 58.5 g of asolid having a product content of 95.4% (HPLC, compared to standard),resulting in a yield of 93.9% of theory; the cis/trans ratio is 71:29.

Example 6

43.2 g [0.45 mol] of ammonium carbonate and 29.4 g [0.6 mol] of sodiumcyanide are initially charged in 560 ml of water. Starting at roomtemperature, 38.5 g [0.3 mol] of 4-methoxy-cyclohexanone are addeddropwise. The reaction mixture is stirred at 55–60° C. for 4 hours,cooled to 0–5° C. and stirred at this temperature for 2 hours. The solidis filtered off with suction and dried. This gives 26.4 g of a solid,resulting in a yield of 44.4% of theory; the cis/trans ratio is>99.7:0.3.

Melting point: 267–268° C. (sublimation).

¹H-NMR (400 MHz, d-DMSO): δ=1.38–1.48 (m; 2H), 1.57–1.68 (m; 4H),1.91–1.95 (m; 2H), 3.14–3.17 (m; 1H), 3.23 (s; 3H), 8.37 (s; 1H) ppm.

Example 7

34.6 g [0.36 mol] of ammonium carbonate and 29.4 g [0.6 mol] of sodiumcyanide are initially charged in 560 ml of water. Starting at roomtemperature, 38.5 g [0.3 mol] of 4-methoxy-cyclohexanone are addeddropwise. The reaction mixture is stirred at 55–60° C. for 4 hours,cooled to 0–5° C. and stirred at this temperature for 2 hours. The solidis filtered off with suction and dried. This gives 18.8 g of a solid,resulting in a yield of 31.6% of theory; the cis/trans ratio is99.4:0.6.

Example 8

28.8 g [0.3 mol] of ammonium carbonate and 16.2 g [0.33 mol] of sodiumcyanide are initially charged in 560 ml of water. Starting at roomtemperature, 38.5 g [0.3 mol] of 4-methoxy-cyclohexanone are addeddropwise. The reaction mixture is stirred at 55–60° C. for 4 hours,cooled to 0–5° C. and stirred at this temperature for 2 hours. The solidis filtered off with suction and dried. This gives 15.5 g of a solid,resulting in a yield of 26.1% of theory; the cis/trans ratio is99.2:0.8.

Example 9

13.5 g [140 mmol] of ammonium carbonate and 1.62 g [33 mmol] of sodiumcyanide are initially charged in 56 ml of water. Starting at roomtemperature, 4.3 g [30 mmol] of 4-ethoxy-cyclohexanone are addeddropwise. The reaction mixture is stirred at 55–60° C. for 4 hours,cooled to 0–5° C. and stirred at this temperature for 2 hours. The solidis filtered off with suction and dried. This gives 5.55 g of a solid(78.8% of theory); the cis/trans ratio is 72:28.

¹H-NMR (400 MHz, d-DMSO): δ=1.09 (t; 3H, cis), 1.12 (t; 3H, trans),1.3–1.48 (m; 2H, cis+trans), 1.57–1.64 (m; 4H, cis+trans), 1.77–1.95 (m;2H, cis+trans), 3.25–3.3 (m; 1H, cis+trans), 3.40 (q; 2H, trans), 3.45(q; 2H, cis), 8.40 (s, br; 1H, cis+trans) ppm.

Further examples of the formula (III)

which may be mentioned are:

Example 10

R¹═O—^(n)C₃H₇ m.p.>250° C. cis/trans=87/13

Example 11

R¹═O—^(n)C₄H₉ m.p.>250° C. cis/trans=85/15

Example 12

R¹═O—^(i)C₄H₉ m.p.>250° C. cis/trans=51/49

Example 13

In an autoclave, 19.8 g [0.1 mol] of4-methoxycyclohexane-1-spiro-5′-hydantoin (cis/trans ratio 71:29), 4 g[0.1 mol] of sodium hydroxide and 400 ml of water are heated at 160° C.for 24 hours. With ice-cooling, the reaction mixture is adjusted to pH 3using hydrochloric acid and concentrated substantially under reducedpressure. The remaining water is removed by azeotropic distillation withtoluene. This gives 29.6 g of a solid.

According to GC/MS (after silylation), 3.7% of starting material and89.3% of 4-methoxycyclohexane-1-amino-carboxylic acid are present; thecis/trans ratio is 70:30.

GC/MS(sil.): m/e=302 (product (disilylated)—15), 200 (base peak, product(disilylated)—CO₂SiMe₃), 168 (200—MeOH).

Example 14

In an autoclave, 7.9 g [40 mmol] ofcis-4-methoxycyclohexane-1-spiro-5′-hydantoin, 160 ml of water and 1.6 g[40 mmol] of sodium hydroxide are heated at 160° C. for 24 hours. Withice-cooling, the reaction mixture is adjusted to pH 3 using hydrochloricacid and substantially concentrated under reduced pressure. Theremaining water is removed by azeotropic distillation with toluene. Thisgives 11.2 g of a solid.

m.p.>400° C.

¹H-NMR (400 MHz, d₆-DMSO): δ=3.17 (m, 1H, CHOCH₃), 3.22 (s, 3H, OCH ₃)ppm.

Example 15

In an autoclave, 1 g [5 mmol] oftrans-4-methoxycyclohexane-1-spiro-5′-hydantoin, 20 ml of water and 0.2g [5 mmol] of sodium hydroxide are heated at 160° C. for 24 hours. Withice-cooling, the reaction mixture is adjusted to pH 3 using hydrochloricacid and substantially concentrated under reduced pressure. Theremaining water is removed by azeotropic distillation with toluene.

This gives 0.8 g of a solid.

Example 16

6.9 g [40 mmol] of cis-4-methoxycyclohexane-1-aminocarboxylic acid aresuspended in 50 ml of anhydrous methanol. The mixture is briefly heatedto reflux and then cooled to 0° C. At 0–5° C., 6.9 g [58 mmol] ofthionyl chloride are added dropwise. The mixture is stirred at 0–5° C.for half an hour, then allowed to warm to room temperature, heated to40° C. and stirred at 40° C. overnight. The reaction mixture isfiltered, the filter residue is washed with 20 ml of methanol and thefiltrate is concentrated. The residue is stirred with 50 ml of methyltert-butyl ether and filtered off with suction, and the residue isdried. This gives 5.6 g of methylcis-4-methoxy-cyclohexane-1-aminocarboxylate hydrochloride (63% oftheory).

m.p. 298° C.

¹H-NMR (400 MHz, d-DMSO): δ=1.64–1.80 (m; 4H), 1.88–1.96 (m; 4H), 3.23(s; 3H), 3.29–3.32 (m; 1H), 3.76 (s; 3H), 8.67 (s, br; 3H) ppm.

Example 17

In the same manner as described in Example 12, methyltrans-4-methoxycyclohexane-1-aminocarboxylate hydrochloride is prepared.

m.p. 173° C.

¹H-NMR (400 MHz, d₆-DMSO): δ=185–2.37 (4 m, 8H, CH₂), 3.32 (s, 3H, CHOCH₃), 3.50 (“d”, 1H, CHOCH₃), 3.82 (s, 3H, OCH ₃), 8.94 (br, 3H, ^(⊕)NH₃)ppm.

Similarly to Example 15, the following amino acid esters of the formula(IV) are obtained

Example 18

R¹═O—C₂H₅ R²=Me m.p.>220° C.

Example 19

R¹═O—^(n)C₃H₇ R²=Me m.p.>220° C.

Example 20

R¹═O—^(n)C₄H₉ R²=Me m.p. 183° C.

Example 21

R¹═O—^(i)C₄H₉ R²=Me m.p. 179° C.

Example 22

R¹═OMe R²=Et MS(silyl.): m/e=273 (M⁺)

Example 23

R¹═OMe R²=^(n)Bu ¹H-NMR

¹H-NMR (400 MHz, d-DMSO): δ=0.88–0.92 (t; 3 H), 1.32–1.41 (m; 2H),1.57–1.68 (m; 2H), 1.69–2.1 (m; 10H), 3.23 (s; 3H), 3.27–3.31 (m; 1H),4.14–4.18 (m; 2H), 8.77 (s, br; 3H) ppm.

Example 24

10.2 g of the compound of the formula (III) where R¹═OR³, R³ beingmethyl (8-methoxy-1,3-diazaspiro[4.5]decane-2,4-dione; 97% pure,cis/trans ratio=75:25) are stirred at 55° C. in 86 ml of water and 9.8 gof 26% strength ammonia for 4 hours. The mixture is cooled to 0–5° C.and stirred at this temperature for 2 hours. The solid is filtered offwith suction and dried. This gives 5.37 g of a solid; the cis/transratio is 98.3:1.7.

Example 25

Example 24 is repeated, except that the mixture is stirred at roomtemperature for 4 hours. This gives 5.03 g of a solid having a cis/transratio of 97.7:2.3.

Example 26

10.2 g of the compound of the formula (III) where R¹═OR³, R³ beingmethyl (8-methoxy-1,3-diazaspiro[4.5]decane-2,4-dione; 97% pure,cis/trans ratio=75:25) are stirred at 55° C. in 86 ml of water and 6.5 gof 26% strength ammonia for 4 hours. The mixture is cooled to 0–5° C.and stirred at this temperature for 2 hours. The solid is filtered offwith suction and dried. This gives 5.73 g of a solid; the cis/transratio is 97.3:2.7.

Example 27

10.4 g of the compound of the formula (III) where R¹═OR³, R³ beingmethyl (8-methoxy-1,3-diazaspiro[4.5]decane-2,4-dione; 95.3% pure,cis/trans ratio=98.2:1.8) are stirred at 55° C. in 17 ml of water and0.69 g of 26% strength ammonia for 4 hours. The mixture is cooled to0–5° C. and stirred at this temperature for 2 hours. The solid isfiltered off with suction and dried. This gives 9.58 g of a solid; thecis/trans ratio is >99.7:0.3.

1. A compound of the formula (I)

in which R¹ represents OR³, and R³ represents C₁–C₄-alkyl.
 2. A compound of the formula (I) according to claim 1 in which R³ represents methyl or ethyl. 